During this check, the engine must be warmed up to normal operating temperature and the battery must be fully charged
1. Before removing the spark plugs, clean the area around them (if available, compressed air should be used, otherwise a bicycle pump or a small brush can be used). The goal is to prevent dirt from getting inside the cylinders during the inspection.
2. Remove all spark plugs from the engine (see chapter Routine car maintenance).
3. Lock the throttle in the fully open position or have an assistant hold the pedal depressed.
4. On carbureted models, disconnect the low voltage wire from the ignition coil. On fuel injected models, disconnect the fuel pump and ignition circuit by removing the main relay (indicated by the arrow).
5. This is to prevent any fuel that may have been injected into the engine compartment from igniting. The main relay is usually located under the hood near the fuse panel, but for the exact location on your model, refer to Chapter On-board electrical equipment system.
6. Install a compression gauge into the hole of spark plug No.1 (cylinder No.1 is located on the radiator side).
7. Crank the engine through at least seven compression strokes using the starter and watch the compressor readings. On a healthy engine, the compression ratio will increase rapidly. A low compression ratio on the first stroke with a gradual increase in subsequent strokes indicates worn piston rings. A low compression ratio on the first stroke that does not increase in subsequent strokes indicates leaking valves or a burnt-out cylinder head gasket (a crack in the cylinder head can also be the cause). Carbon deposits on the inside surfaces of the valve heads can also cause low compression. Record the highest reading.
8. Repeat the operation for the remaining cylinders and compare the results with the compression ratio values in Table of sizes and adjustment data at the end of the Guide.
9. If the compression ratio is low, pour a small amount of engine oil into each cylinder through the spark plug holes (by pressing the plunger type oiler approximately three times) and check again.
10. If the compression ratio has increased after adding oil, the piston rings are definitely worn. If the compression ratio has not increased significantly, there is a leak in the valves or through the cylinder head gasket. Valve leakage can be caused by burnt valve seats and/or chamfers, as well as warped valves and the presence of cracks and dents.
11. If two adjacent cylinders have the same low compression ratio, then it is likely that the cylinder head gasket between them has burned out. The presence of coolant in the combustion chambers or crankcase will confirm this suggestion.
12. If the compression ratio in one of the cylinders is 20% lower than in the others, and the engine runs slightly unevenly at idle; then the exhaust cam of the camshaft may be worn out.
13. If the compression ratio is unusually high, the combustion chambers are probably covered with carbon deposits.
14. In this case, you should remove the cylinder head and remove the carbon deposits.
15. If the compression ratio is below normal or varies greatly between cylinders, it is a good idea to have a leak test performed at a workshop. This test will accurately indicate the cause of the leak and its extent.